A new report has found that the percentage of U.S. high school students who have tried e-cigarettes has doubled in the past year. The report, written by the Centers for Disease Control and Prevention (CDC) and the Food and Drug Administration (FDA), notes that as of 2012, an estimated 1.78 million students had given electronic smokes a spin.

Analysts estimate that e-cigarette sales in the U.S. will reach $1 billion this year, and predict that sales could hit $10 billion in five years.

E-cigarettes were introduced to the U.S. market in 2007 as an alternative to tobacco cigarettes. Many of them are designed to look like conventional cigarettes, but they are actually battery-operated devices. They contain an atomizer that heats a nicotine liquid that turns to vapor; the vapor is then inhaled and exhaled, much like tobacco smoke.

The liquid — commonly known as e-juice, e-liquid, smoke juice and cig juice — comes in a cartridge and is a mixture of nicotine, water, glycerol, propylene glycol and flavorings, according to the Tobacco Vapor Electronic Cigarette Association.

The amount of nicotine varies; consumers can select cartridges ranging in strength comparable to ultralight cigarettes all the way to regular-strength smokes.

And yes, the liquid comes in flavors. Although e-cigarette manufactures say they don’t market to young people, liquid cartridges come in a nauseating array of flavors — everything from butterscotch and fruit punch to cinnamon bun and milkshake.

Manufacturers also claim that e-cigarettes are a safe alternative to conventional cigarettes. However, in 2009 the FDA announced that a laboratory analysis of electronic cigarette samples found that they contain “carcinogens and toxic chemicals such as diethylene glycol, an ingredient used in antifreeze.”

Regulation has been slippery. According to the FDA, currently e-cigarettes that are marketed for therapeutic purposes (like the cessation of smoking) are regulated by the FDA Center for Drug Evaluation and Research (CDER). But they don’t have jurisdiction over the rest of the market. The FDA has stated its intent to issue a proposed rule that would extend the agency's authority to products that meet the statutory definition of “tobacco product,” which would include e-cigarettes.

Some countries, including Australia, Canada, Israel and Mexico, have banned electronic cigarettes, while other countries have varying degrees of regulation. For now in the States, we only have these words of warning from the FDA: “As the safety and efficacy of e-cigarettes have not been fully studied, consumers of e-cigarette products currently have no way of knowing whether e-cigarettes are safe for their intended use; how much nicotine or other potentially harmful chemicals are being inhaled during use; or, if there are any benefits associated with using these products.”

“Additionally,” they add, “it is not known if e-cigarettes may lead young people to try other tobacco products, including conventional cigarettes, which are known to cause disease and lead to premature death.”

Or as the Mayo Clinic puts it, "until more is known about the potential risks, the safe play is to say no to electronic cigarettes."